1. Field of the Invention
This invention relates to a semiconductor integrated circuit in which electronic circuits constituting a CPU, a memory and so on are formed on a semiconductor substrate while the electronic circuits are separated by their functions to form a plurality of function blocks, and particularly to a semiconductor integrated circuit capable of preventing power supply noise produced as a result of the operation of these function blocks from leaking to the outside.
2. Related Art
Japanese Patent application Laid-open No.60-161655 proposes a semiconductor device wherein a power supply line inside an integrated circuit (hereinafter, referred to as `IC`) is used as one electrode and a diffusion layer formed in a semiconductor substrate along this power supply line is used as another electrode to form capacitance therebetween. The power supply line is formed on the semiconductor substrate through an insulating layer and connected to a ground line. The semiconductor substrate is supplied with a power source voltage by way of a power source line. Therefore, capacitance is provided between the power source line and the ground line. This capacitance is utilized as a part of a bypass capacitor. In this proposed device, because it is possible to provide a bypass capacitor directly on the power supply line inside the IC, fluctuations in power supply line potential occurring at times of on/off transition of transistors inside the IC (i.e. power supply noise) can to some extent be suppressed.
However, just by simply providing a bypass capacitor along the power supply line inside the IC as in the proposed device, it is not possible to certainly prevent the power supply noise generated inside the IC. As a result, it is not possible to fully solve problems such as high-frequency noise leaking from the IC to the outside and the malfunctioning of circuits inside the IC caused by the power supply noise. In more detail, in an IC, generally, numerous function blocks such as a CPU block, an oscillating circuit block, a memory block, an input-output interface block, and so on are disposed dispersed on a semiconductor substrate. These function blocks each constitute a generating source of power supply noise. Consequently, even if a bypass capacitor is just formed along a power supply line connected to the dispersed function blocks as described above, when being seen from a function block having produced power supply noise, capacitance is also dispersed along the power supply line. As a result, with influence of the impedance of the power supply line, the capacitance cannot fully absorb the power supply noise. Therefore, the power supply noise generated in one function block is inputted into other function blocks through the power supply line or signal lines and has an adverse influence on the operation of these other function blocks.
If the capacitance of a bypass capacitor disposed on the power supply line is made large enough, it is possible to suppress the occurrence of power supply noise. In this case, however, to obtain the capacitance necessary for suppression of the power supply noise it is necessary to increase the capacitance taking into account the influence of the impedance of the power supply line. Therefore, capacitance which is considerably large must be formed to suppress the power supply noise, and consequently the rate of occurrence of defects rises and yield deteriorates.
That is, to reduce power supply noise by providing bypass capacitors on the power supply line while maintaining the reliability of the IC it is necessary to provide the capacitors of the right capacitance in the right location, and just disposing bypass capacitors at random as in the proposed device as mentioned above is not sufficient.